I can think of several, which problem were you thinking of?

In lots of countries you need a specific right to work, and people who are on holiday visas or who are making asylum applications, or have simply entered the country without the right to do so, are not allowed to work.

Some consider these restrictions themselves to be a problem.

Currently, employers in the UK are legally required to check the right-to-work status of people they employ. This is usually done with a random assortment of ID documents and visa status checks. The proposal (I think) is to replace this and other functions with "Britcard", a digital ID system.

So another problem might be that government security schemes are usually pretty bad.

And a further one could be that there's little to stop (say) an asylum applicant from 'borrowing' someone else's britcard-enabled phone to sign on and work Uber Eats illegally, which is one of the issues that they are allegedly trying to tackle.

Beyond that ... sure there's massive privacy implications etc etc.

So yeah, which problem did you have in mind?